Photoconductive target electrode for a pickup tube and its method of fabrication



R. A. S IMMS 3,303,344 IHOTOCONDUCTIVE TARGET ELECTRODE FOR A PICKUP Feb. 7, 1967 TUBE AND ITS METHOD OF FABRICATION Filed July 16, 1963 VOLTAGE SOURCE Fig. l

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United States Patent M 3,303,344 PHOTOCONDUCTIVE TARGET ELECTRODE FOR A PICKUP TUBE AND ITS METHOD OF FABRI- CATION Robert A. Simms, Stamford, Conn, assignor to Westinghouse Electric Corporation, Pittsburgh, Pa, a corporation of Pennsylvania Filed July 16, 1963, Ser. No. 295,450 Claims. (Cl. 25fi21l) This invention relates to a light sensitive device and more particularly to a photoconductive device.

One particular application of this photoconductive target is in a pickup tube known as a vidicon. The vidicon is compressed of an evacuated envelope, The envelope is provided with a face plate or window portion transmissive to the input radiation from the scene, An electrically conductive film is provided on the inner surface of the face plate. The conductive coating is the signal plate or back plate of the target and a layer or layers of photoconductive material are provided on the conductive film to complete the light sensitive target structure. An electron gun is provided at the opposite end of the envelope with respect to the face plate for generating and directing an electron beam onto the target member. Suitable scanning and focusing means are provided either within or exterior to the envelope for focusing and scanning the electron beam over the target. The cathode electrode of the electron gun is normally held at a negative potential with respect to the signal plate of the target. The electrons within the scanning beam may be of a high energy, that is above the first cross-over potential and below the second cross-over potential of the target. The electron beam may also be of the low velocity type scan in which the energy of the electrons is below the first cross-over potential of the target. The most common type of operation is the utilization of the low velocity type of scan. This invention is directed primarily to a target structure for use in a low velocity type scan operation.

In the operation of the low velocity type scan vidicon type pickup tube, the electron beam is scanned across the target and electrons are deposited on the surface of the photoconductive material of the target so that the exposed or scanned surface is charged to substantially the potential of the cathode of the eiectron gun. The back plate electrode of the target is maintained at a positive potential with respect to the cathode of the electron gun. When light or any other suitable radiation is focused upon the target, it renders the photoconductive material conductive in that particular area and causes a corresponding portion of the exposed or scanned surface of the photoconductive layer to discharge toward the potential of the back plate electrode, The next time that the electron beam scans this area it again restores or charges the surface to cathode potential. The return of the area to cathode potential restores the original volt- 7 age difference across the photoconductive layer and causes an electron current to flow in the signal plate. The electron current which flows in the signal plate is coupled to an output circuit to derive an electrical signal representative of and corresponding to the light directed on the area of the photoconductive layer.

The scanning rate in a conventional vidicon utilized in the television industry is such that each elemental .area is read by the scanning electron beam thirty times per second. Several different types of photoconductive materials have been used in this application such as selenium and antimony trisulfide. These materials have operated satisfactorily in television applications where the scanning rate is thirty times per second. In other Patented Feb. 7, 1967 applications, however, it is desirable to scan at a slower rate such as ten times per second or less. This places certain requirements on the target structure. In general, the target must have the ability to retain a charge during the time the electron beam is scanning the remaining elements of the target. This requires that the target should have a low dark current to prevent leakage and decay of the signal during the time the electron beam is scanning the remaining elements and the lateral leakage of the target must also be of a very low value to prevent degradation of resolution while the storage and reading operations are performed. The advantage of using a slow scanning rate in a pickup tube in certain applications is to increase the signal to noise ratio of the output and also to reduce the required band pass of the television system. The longer the light remains on the target the greater the discharge and therefore the greater the signal output.

It was found that the antimony trisulfide type of surfaces were not desirable for slow scan applications because low resistivity and in some applications low sensitivity and poor uniformity due to difficulties involved in evaporating the material. These characteristics reduce the capabilities of the surface to retain or build up a charge representing a signal during the long frame time of a slow scan operation. Since the light resistance of the antimony trisulfide is relatively high, the surface will exhibit only a small positive change in potential when light falls on it. Since the dark resistance of the antimony trisulfide is relatively low, a similar positive charge will appear on the surface due to electrons leaking through the surface over a long frame time. The difference in potential over the surface will, therefore, be very small due to the input signal and thereby resulting in a very low sensitivity target.

Photoconductive targets manufactured from selenium, on the other hand, are very sensitive to light and a'large charge can be built up across such a surface. The dark current in the selenium type target is also much lower than antimony trisulfide and the charge may be retained over a longer frame time. The selenium target has disadvantages, however, which limit its application to slow scan and conventional scan operations. The selenium surface is a very unstable surface and at a temperature slightly above F. the target tends to become conductive even in the dark. The selenium is normally evaporated to form an amorphous type deposit. It is found that this amorphous form of selenium has a tendency to revert to a crystalline state which has a much lower resistance. It has also been found that in the operation of a selenium target that if the voltage is raised to a critical potential only slightly above the operating range, conducting spots occur which appear as bright spots in the resulting picture. This is believed due to local crystaliization within the selenum layer. Another disadvantage of selenium is the effect wherein the scan surface may become positively charged with respect to the signal electrode. This is believed to be due to secondary emission from the selenium surface exceeding unity, that is, more secondary electrons leaving the surface than land on it. This results in charging the surface of the selenium to the potential of the nearby collector and accelerating electrode which is of the order of hundreds of volts positive with respect to the electron gun cathode. This positive charging of the selenium surface causes the dark current to reverse its direction and no signal is obtained from the signal plate in response to light directed thereon.

In accordance with the above teaching, US. Patent 3,020,442 entitled, Photoconductive Target, by J. F. Nicholson et al. combined the properties of the amorphous selenium and antimony trisulfide layers in order to obtain a very good slow scan vidicon type tube. The composite target as described in the above-mentioned patent produced a very sensitive target and one of low dark current over a conventional antimony trisulfide target. It was found under normal television scanning operation that the signal output current was increased and the dark current decreased to about to of its value over a conventional antimony trisulfide target. The sensitivity of this device was such that with a target potential of volts and an input of .6 foot candles the video signal output was about .04 micro-amperes and the dark current was about .001 micro-amperes.

In order to provide for the ever increasing demand of a more sensitive vidicon, the present invention provides that at similar target potentials and input illumination a signal output of .45 micro-amperes with a dark current of .009 micro-amperes can be obtained.

It is, accordingly, an object of this invention to provide an improved photoconductive target.

It is another object to provide an improved photoconductive target of high sensitivity for a pickup tube.

It is another object to provide an improved photoconductive target for use in slow type scan operations.

It is a further object of this invention to provide an improvement in photoconductive targets of the type consisting essentially of selenium.

Briefly, the present invention accomplishes the above objectives by providing a layer of selenium doped with a small amount of antimony to provide an impurity level yjvhigh is closer to the valence band than the conduction These and other objects of the invention will become more apparent when considered in view of the following specification and drawing, in which:

FIGURE 1 is a view in section of a cathode ray camera tube using a photoconductive target according to the teaching of this invention;

FIG. 2 is an enlarged sectional view of the target shown in FIG. 1;

FIG. 3 illustrates the response curves of a layer composed of selenium and antimony trisulfide and a layer composed of doped selenium and antimony trisulfide indicating the signal output with response to the spectral ama FIG. 4 illustrates a family of response curves of a target com rised of antimony trisulfide and selenium compared with a target comprised of antimony trisulfide and doped selenium under different operating conditions in which the face plate illumination is plotted with respect to the signal output.

Referring in detail to FIGS. 1 and 2, there is illustrated a vidicon type pickup tube embodying my invention. The tube is comprised of an evacuated envelope 12 containing an electron gun 2t) and a target member 30. Suitable deflection and focusing means for the electron beams and illustrated here as coils are provided around the envelope. The envelope 12 includes a light transmissive face plate portion 14 of a material such as glass which is coated on its inner surface with a suitable light transmissive electrical conductive film 32. This conductive film 32 may be of a suitable material such as tin oxide. The film or coating 32 is the signal electrode or back plate of the tar- .get 30 and an electrically conductive lead-in member 34 is provided to the exterior of the envelope 12. The leadin 34 is connected through a resistor 35 to a suitable volt- .age source 37. The signal output is derived from the resistor 35. The photoconductive material is deposited as a thin layer upon the electrical conductive film 32. In the specific embodiment shown, the photoconductive material layer 36 is comprised of two separate and distinct layers 42 and 44. A layer 42 of doped selenium is deposited on the conductive layer 32 and a layer 44 of antimony trisulfide is deposited on the exposed surface of the doped selenium layer 42.

An electrically conductive grid or mesh 38 is provided adjacent the target 30 and provides a uniform de-accelerating field for the electrons approaching the target 30. The mesh 38 is normally opera-ted at a potential of about 250 volts positive with respect to the cathode 22 of the electron gun 20. The back plate 32 is normally operated at a potential of about 15 volts positive with respect to the electron gun cathode. The voltage source 37 provides the potential for the back plate 32.

In preparing targets in accordance with this invention, the doped selenium material for the layer 42 is prepared by taking weighted amounts of high purity selenium such as 99.999% pure selenium pellets, Grade A-58, obtained from the American Smelting and Refining Company and adding to this 0.1 weight percent excess of high purity antimony such as 99.99% pure zone refined antimony, Grade A-60, obtained from the same source. The materials are weighed on an analytical balance to five significant figures. They are then mixed together, placed in a 9 millimeter outside diameter Vycor or quartz bulb, evacuated to approximately 10- torr and sealed off. An inert gas such as argon may be used to fill the bulb after evacuation. The bulb is then placed in a tube furnace and heated to 300 C. for 15 minutes. It is then air quenched to room temperature and the bulb broken open. About 475 milligrams of this doped selenium is then placed in a tantalum boat and the boat and glass support member 14 having the conductive coating 32 thereon are all placed in a closed container capable of being evaporated and the container is evaporated to a pressure of about 10 torr. The boat with the doped selenium is then positioned at a throw distance of about 7 inches from the layer 32. At the beginning of the heating cycle for the boat, the doped selenium is shuttered until a rapid evaporation rate is efiected. The shutter is then removed and the evaporation is continued until approximately all of the material in the boat is evaporated onto the target. At this point the tantalum boat is again shuttered. This evaporation normally takes place in about four minutes, and the boat should be heated to a temperature of about 250-500 C. After the doped selenium layer 44 has been deposited on the target the second layer 44 of antimony trisulfide may be evaporated. Here again, the boat containing the antimony trisulfied material is positioned within the container about 7 inch flow distance from the target and the boat is heated to a temperature of 400 to 500 C. to deposit about 50 milligrams of antimony trisulfide onto the target. The heating step is performed in a vacuum of about 10- torr. The resulting structure provides a layer 42 of a thickness of about 0.5 to 1.0a and a layer 44 of a thickness of about .OS/L t0 0.5 1"

The system should be allowed to pump for approximately 15 minutes after evaporation of the layers 42 and 44. The target 30 is then removed from the container and placed inside the conventional vidicon envelope and evacuated.

The next process in preparing the target 30 may be referred to as the diffusion process. This is one of the more critical processes involved in the final fabrication. The tube should be set up under conventional operating voltages with the target at 15 volts and a light input of about .6 foot candles and the video signal and dark current should be checked. The target voltage should be left at 15 volts and the focus current coil slightly changed to obtain defocusing. A watt 2870 tungsten light source is then positioned about 3 feet from the target member so as to provide uniform illumination and heating of target 30. Under these conditions, the target current should read about .5 to .8 micro-amperes and the face plate temperature should be at a temperature of about 36 C. This diffusion process should be carried on for a few hours with continual monitoring of the sensitivity of the device.

As the diffusion process progresses and more antimony finds its way into the selenium, the impurity level builds up, which results in the necessary decrease in the magnitude of energy necessary to create charge carriers. The direct result of this is the fact that although the photosensitivity increases, the thermal sensitivity or dark current tends to increase at a faster rate with the result that the video signal to dark curent ratio decreases. Side effects of this decreased ratio results in an increase in random noise which is intolerable, and a decrease in the discharge time constants which, though desirable, must be sacrificed in view of the random noise problem. Therefore when the desirable sensitivity and dark cur-rent is obtained by proper readings the diffusion step should be stopped.

The above diffusion process is only one method of sensitizing the target and other methods may be utilized. The important thing is to effect a diffusion process at the lowest possible temperature. This requirement of an optimized gradient should provide a satisfactory diffusion at temperatures below those critical to crystallization of the selenium material. This is accomplished in the above method by doping the selenium with an amount of antimony of .01 to 1.0% by weight and shuttering the evaporation of both layers to provide the maximum density at the doped selenium and the antimony trisulfide interface. The amount of dopant of course is chosen to provide the optimum concentration gradient. This means that the amount of dopant should be kept within the range of .01 to 1% and the value of .l% was found to be the most desirable figure.

The target constructed in accordance with the above teaching was compared with a selenium target having an antimony trisulfide layer. These tests show the improved properties. The target provides a vidicon whose signal to noise ratio at normal light levels will be an order of magnitude higher than any known device and is capable of slow scan. In addition, the spectral band width of the device will be approximately from 12 00 Angstroms to 7200 Angstroms. The target also has excellent discharge and rise time functions.

In FIG. 3, a comparison between a target composed of a layer of selenium and a covering layer of antimony trisulfide (curve 50) and the structure described herein (curve 52) is illustrated.

In FIG. 4, a comparison is made between the same targets as FIG. 3 with the family of curves 54 illustrating the selenium and the family of curves 56 illustrating the doped selenium described herein for different values of target voltages.

While I have shown the invention in only one form, it will be obvious to those skilled in the art that it is not so limited but is susceptible of various other changes and modifications without departing from the spirit and scope thereof.

I claim as my invention:

1. A photoconductive target electrode for a pickup tube, said target electrode comprising a transparent elec- 6 trically conductive film, a thin film of amorphous selenium doped with .01 to 1 percent by weight of antimony deposited on one surface of said conductive film and an antimony trisulfide coating on said film of amorphous selenium.

2. A photoconductive target electrode for a pickup tube, said target electrode comprising an electrically conductive film, a thin film of amorphous selenium on one surface of said conductive film and having from .01 to 1 percent by weight of antimony distributed throughout said selenium layer and a layer of antimony trisulfide on the exposed surface of said amorphous selenium film, said conductive film operating at a positive potential with respect to the exposed surface of said antimony trisulfide film.

3. The method of fabricating a photoconductive target for a pickup tube comprising evaporating a mixture consisting of selenium and having intimately and substantially uniformly distributed therein from .01 to 1 percent by weight of antimony to form a first layer, evaporating a second layer onto said first layer comprising the step of evaporating a layer of antimony trisulfide and heating said target to a temperature of about 36 degrees centigrade to provide inter-diffusion.

4. The method of making a light sensitive target for an electron discharge device which comprises, depositing a film of electrically conductive transparent material onto a support member, evaporating a quantity of photoconductive material onto said film of electrically conductive material, said photoconductive material comprised of selenium having from .01 to 1 percent by Weight of antimony distributed throughout said selenium material, evaporating a second photoconductive material in a high vacuum onto said selenium mixture layer to form a photoconductive surface and then treating said target whereby inter-diffusion occurs between said layers.

5. A photoconductive target electrode for a pick-up tube, said target electrode comprising a light-transmissive electrically conductive layer, a first photoconductive layer deposited on said electrically conductive layer, said first photoconductive layer of selenium material and having from .01 to 1 percent by weight of antimony distributed throughout said photoconductive layer and a second photoconductive layer deposited on one surface of said first photoconductive layer.

References Cited by the Examiner UNITED STATES PATENTS 2,378,513 6/ 1945 Thompson et al. 117-200 2,687,484- 8/1954 Weimer 338--17 X 2,994,621 8/1961 Hugle et al. 252-501 X 3,020,442 2/ 19621 Nicholson et al. 252-501 X 3,046,431 7/ 1962 Nicholson 313-6S RALPH G. NILSON, Primary Examiner.

WALTER STOIJWEIN, Examiner. 

1. A PHOTOCONDUCTIVE TARGET ELECTRODE FOR A PICKUP TUBE, SAID TARGET ELECTRODE COMPRISING A TRANSPARENT ELECTICALLY CONDUCTIVE FILM, A THIN FILM OF AMORPHOUS SELENIUM DOPED WITH .01 TO 1 PERCENT BY WEIGHT OF ANTIMONY DE- 